It is well known to determine the contents level of a fluid in a container with a level measurement apparatus that uses electromagnetic measurement signals which are guided by a wave guide being submerged into the fluid. These measurements are also known for microwave signals as Time Domain Reflectometry (TDR) level measurements. According to this measurement principle measurement signals are created and processed in the level measurement apparatus that is usually mounted on the top of the container. The measurement signals are given on a wave guide that is placed inside the container and submerged into the fluid or medium stored in the container. The determination of the contents level is based on measuring the time of flight of the measurement signals from the measurement apparatus along the wave guide to the fluid in the container and from there back to the level measurement apparatus. The measured time of flight of the signals is a measure for the distance from the measurement apparatus to the surface of the fluid whereby the propagation of the measurement signals along the wave guide depend on the dielectric constants of the media surrounding the wave guide in the container. In consideration of predetermined geometrical data of the container the contents level of the fluid can be calculated.
As far as a contents level of a single fluid is measured in the container the aforementioned common measurement methods are very reliable. Problems occur when a second fraction or phase separates from the fluid such that finally two fluid volumes can be found in the container. If for example an emulsion of oil and water separates more and more in the container whereof the water fraction is small compared to the volume of the oil fraction. Most of the common TDR level measurements would not even recognize a small water fraction and if they do an exact dielectric constant of the water fraction must be known and used in calculating the volumes of the water and of the oil fractions. This usually ends up in a recalibration of the TDR level measurement instrument concerned.